Trump can win the GE. Many people who wouldn't normally vote, who see the whole system as corrupt, will vote for him.
Millions of people see liberals winning time and time again, sees Republicans as capitulating time and time again, moving farther left every cycle. They're sick of it.
Last time around Republicans thought polls were underestimating Romney when really they were pretty accurate in predicting Obama's comfortable victory. This time around, with Trump being so unconventional and attracting people to vote who normally wouldn't, you can expect polls that have traditional models of likely voters to underestimate him.
That isn't to say he's a lock to win, but he can win. I can't imagine who in the world got excited for Romney, and he won over 45% of the popular vote. And Clinton/Sanders/Biden are certainly less galvanizing politicians than Obama is.
Trump can win this way. This is how Jesse Ventura won the Governor's race in MN in 98. He won because he attracted voters that never voted before. Now that the idea of a GOP brokered convention being a plausible scenario, Trump will stay, Republican or not.
